Star Trek: Tribbles

Federation research has shown,
That tribbles near the Neutral Zone,Have main bodies so small,They’re not visible at all;
Why their nutsacks are so big is unknown.

Tribbles are fictional furry pets that breed prodigiously. The were featured in the original Star Trek episode “The Trouble With Tribbles” in 1967. Some 1,500 were constructed for the episode. See Wikipedia for more info.